The use of microcapsules is becoming increasingly common as a method for protecting and controlling the release of active ingredients. Microcapsules are used in a number of industries including agrochemicals, detergents, personal care products and pharmaceuticals. Core-shell capsules, in which droplets of the liquid active ingredient or solution are surrounded by a solid spherical shell, usually made of a polymer, can be prepared by a process of interfacial polymerization at the surface of the droplet. In this process, an emulsion (normally of the oil-in-water type, in which the active is present in the oil phase) is made, and the polymer shell is grown around the outside of the droplets.
There are many different types of chemical reactions that can be used for this process, and of these two have become common commercially. In the first, a di- or polyfunctional isocyanate compound is dissolved into the oil phase, and this solution is then emulsified under high shear conditions using a surfactant, water soluble polymer or ‘protective colloid’ to stabilize the droplets that are formed. The isocyanate then either reacts with the surrounding water to form a polyurea polymer at the droplet surface—a process that is relatively slow and requires heating and/or the use of a catalyst—or a water-soluble second reactant such as an amine or a polyfunctional alcohol is added to the water which reacts with the isocyanate to form a polyurea or polyurethane polymer respectively. Examples of these processes can be found in U.S. Pat. No. 3,577,515 and U.S. Pat. No. 4,285,720.
In the second commonly-used chemistry, the capsule wall is formed by the condensation of an amine (such as urea or melamine) or a phenolic compound with formaldehyde. This process was first described in the 1960s (see for example GB989264). All of the components are soluble in water, but the polymerization process is preferentially initiated on the surface of the emulsified droplets. Whilst this is a successful method of producing capsules, it can be difficult to remove traces of formaldehyde from the products, which can be undesirable.
Double-walled capsules with a polyurea inner shell and a urea-formaldehyde outer shell have been reported recently by different authors (Gang Li et al, Polymer Bulletin 60, 725-731 (2008); Caruso, M. M. et al, Applied Materials and Interfaces 2, (4), 1195-1199 (2010)). As with conventional urea-formaldehyde capsules, these rely on the tendency for urea-formaldehyde condensation to nucleate on the surface of the droplets or capsules. However, as mentioned above, it is generally desirable to avoid the use of formaldehyde.
WO-A-2009/063257 describes a process in which the surfactant used to stabilize an emulsion prior to the formation of microcapsules is replaced by mineral particles, so that a ‘Pickering’ emulsion is formed in which the particles are located at the oil/water interface. The examples within it describe the formation of capsules using an oil phase containing a polyisocyanate as the first reactant, and either an amine, an alcohol or water as the second reactant to form the capsule wall. The mineral particles become embedded in the polymer shell as polymerization proceeds. Such capsules have advantages over surfactant-stabilised, polymer-only capsules:                The use of mineral particles leads to a narrower size distribution emulsion which is also resistant to Ostwald ripening, a process in which the small solubility of the oil in water allows it to transfer between droplets, causing large droplets to grow and small droplets to shrink.        The size of the droplets can be controlled to a certain extent by the amount of mineral particles added.        The interior of the capsules is protected from ultraviolet degradation by light scattering from the particles in the shell.        The capsules can be dried without sticking or coalescing with each other.        
The primary field of interest in WO-A-2009/063257 is the encapsulation of agrochemicals. It would advantageous to apply this technology to other applications outside of the field of interest of agrochemicals. One such application is the encapsulation of fragrance ingredients in detergents and personal care products. Such products are typically encapsulated commercially using the urea/melamine formaldehyde process described above.
Whilst some fragrances can be incorporated into leak-resistant capsules using the process described in WO-A-2009/063257, there are a number of ‘difficult to encapsulate’ formulations in which the leakage rate from the capsules in either alcohol or surfactant solutions is unacceptably high. Intact capsules are observed to form around these formulations, but it is believed that the capsule shells are too permeable to the core material in order to be effective. A method is therefore required by which the microcapsules can be made less permeable to a wider range of core ingredients.